The presence of alkali metal ions, particularly sodium ions, is detrimental to the functioning of metal-on-silicon (MOS) devices. Alkali metal ions diffuse rapidly, particularly through silicon dioxide. Sodium ions, for example, are mobile in silicon dioxide even at room temperature. Because they carry an electric (positive) charge, the presence of alkali metal ions will cause the electrical characteristics of a device to drift, with a potentially substantial loss of control of critical functions. Typically, a significant concentration of contaminating alkali metal ions will change the surface potential of an insulator such silicon dioxide resulting in a material loss of stability of the device.
It is recognized that, as a practical matter, it is impossible to completely eliminate alkali metal ion contamination from a device or structure. Such contamination may be minimized to a degree, however, by incorporating into the device or structure certain materials, such as phosphosilicate glass, which getter or chelate alkali metal ions. Such materials, however, are only effective in removing limited amounts of alkali metal ion contaminants. As it is very difficult to remove these contaminants, it will be appreciated that a method of conveniently detecting the presence of unacceptably high concentrations thereof at an early stage in device manufacture is of considerable advantage in saving expensive, time-consuming processing operations. Such a method is provided in accordance with this invention.